The invention relates to a method and a measuring apparatus for determining specific quantities for the gas quality or energy consumption in domestic and industrial areas.
The (natural) gas composition and thus the gas quality will fluctuate more frequently and strongly in future due to new sources of origin (biogas, liquefied gas from all regions of the world, hydrogen from the exploitation of excess current from the generation of alternative energy) and will thus have different effects on gas application processes, which may also include such that are negative. The processes could be adjusted to the fluctuating gas quality by direct measurement of specific quantities for the on-site gas quality in order to ensure optimal and secure operation. The specific quantities for gas quality include e.g. the Wobbe index for burner control units, the air-to-fuel ratio in power generation systems (industrial furnaces, fuel cells etc.), the methane number for gas motors, or the calorific value for billing the purchased energy quantity. The latter requires however that the purchased gas quantity is measured, which currently occurs, apart from a few exceptions, by means of volumetric flow measurements with diaphragm gas meters (domestic) or, in the case of purchasers of large gas volumes (industry), by means of rotary displacement meters, turbine wheel meters, or ultrasonic flow meters. All these measuring means are only suitable for determining the operating volume. In order to draw conclusions from these data on the purchased, billable energy, it is necessary to provide a conversion to standard volume and also to have information on the calorific value of the respectively supplied gas. Both occur only imprecisely: the standard volume is usually calculated with a mean temperature and a mean pressure, and the calorific value is also a value averaged over the billing period.
A method is known from the patent application EP 14001767 in which a flow is generated by a critical nozzle in order to determine specific quantities for the gas quality by means of a downstream microthermal sensor. Said method requires that critical pressure conditions prevail at all times via the nozzle, either by supplying the nozzle with an upstream pressure or by generating a vacuum behind the nozzle. The method is therefore not directly suitable for determining specific quantities for the gas quality at the end customer, because the supply networks at this point rarely have the required upstream pressure and the installation of a vacuum pump behind the nozzle is out of question.
A method is known from EP 2 574 918 A1, in which a microthermal sensor is used to upgrade a volumetric flow measuring device in the respect that the thermal diffusivity can be determined, which in the case of known thermal conductivity allows a classification of the gases into L(ow calorific) or H(igh calorific) gases. It is, however, not possible to draw conclusions with sufficient precision on the calorific value and the energy flow from the volumetric flow, thermal diffusivity and thermal conductivity.